lin-4 and let-7 encode two small (21-25nt) RNAs that are required for the coordination of developmental timing in Co elegans. Both are temporally expressed and function as translational repressors by binding to complementary sequences found in the 3' UTRs of target RNAs. It has recently been discovered that these small RNAs represent the founding members of a new class of evolutionarily-conserved genes, termed microRNAs. At the present time, only two C. elegans RNA-binding proteins, ALG-1 and ALG-2, have been implicated in the processing and function of l/n-4 and let-7 RNAs. Experiments outlined in this proposal will elucidate the relationship between ALG-1/ALG-2 and lin-4, let-7 and other microRNAs. Specifically, a high-throughput RNAi screen will be employed to identify other C. elegans genes that require alg-1/alg-2 for function and will determine other regulatory/developmental pathways that require alg-1/alg-2 for efficient regulation. This genetic strategy will be complemented with molecular/biochemical techniques to identify additional microRNAs that require alg-1/alg-2 for processing and other proteins/RNAs that associate with the ALG-1/ALG-2 ribonucleoprotein complex.